In the production of petroleum, the well fluid often contains considerable quantities of water. In a last production phase from a well it is not uncommon for water to represent up to 90% of the well flow.
Water and petroleum are separated in several steps after the well fluid has flowed out from the well. Before this separation can take place and also between the various steps it is necessary, however, to control the flow rate and the fluid pressure.
This type of control is typically performed by means of a control valve positioned in the flow path in front of the process step in question.
A theoretical description of a separation process between non-mixable fluids is usually based on Stoke's law:
      V    t    =                    g        ·                  D          2                ·                  (                                    ρ              1                        -                          ρ              2                                )                            18        ·        μ              ⁡          [              m        s            ]      
For oil/water separation Vt is the rate of ascent of an oil droplet in water, g is gravity, D is droplet diameter, ρ1 is specific gravity of the continuous phase (water), ρ2 is specific gravity of the dispersed phase (oil) and μ is the viscosity of the continuous phase.
Stoke's law shows that the theoretical efficiency of a separation process increases by the square of the droplet size in the dispersed phase. It is thus of considerable significance that the control valve to the least possible degree reduces the droplet size of the phases in the fluid flowing therethrough.
Prior art control valves inflicts relatively large shear forces on the fluid. The reason for this is, among other things, that the energy loss rate is large, i.e. the fluid energy, e.g. in the form of speed and pressure energy, is lost too quickly. Fluids flowing through prior art choke valves show a substantial reduction of the droplet size(s) in the dispersed fluid phase(s).